Recently, the ecological problems resulted from fossil fuels such as petroleum and coal have been greatly aware all over the world. Consequently, there are growing demands on clean energy. Among various alternative energy sources, a solar cell is expected to replace fossil fuels as a new energy source because it provides clean energy without depletion and is easily handled. A solar cell is a device that converts light energy into electrical energy. The procedure of turning solar energy into electrical energy is called the photovoltaic (PV) effect.
Hereinafter, a conventional process of manufacturing a solar cell is illustrated as follows with reference to FIGS. 1A˜1H.
First of all, as shown in FIG. 1A, a p-type semiconductor substrate 10 is provided. Then, concave and convex patterns with a minute pyramidal shape called as a texture are formed on the surface of the semiconductor substrate 10 in order to improve light absorption and reduce light reflectivity. The texture structure is very minute and thus not shown in FIG. 1A.
Next, as shown in FIG. 1B, an n-type dopant source diffuses into the substrate by thermal diffusion at high temperature, thereby forming an emitter layer 11 on the light-receiving side S1 and a p-n junction interface between the p-type semiconductor substrate 10 and the emitter layer 11. At the same time, a phosphorus silicate glass (PSG) layer 12 is formed on the emitter layer 11.
Next, as shown in FIG. 1C, the PSG layer 12 is removed to expose the emitter layer 11 by an etching procedure. Then, an anti-reflective coating (ARC) 13, which is made of for example silicon nitride (SiN), is formed on the emitter layer 11 in order to reduce light reflectivity and protect the emitter layer 11, as shown in FIG. 1D.
Next, as shown in FIG. 1E, the ARC 13 is selectively removed to expose parts of the emitter layer 11 by an etching procedure. Then, a second thermal diffusion is implemented to form heavily doped n+ semiconductor regions 11′ on the exposed emitter layer 11, as shown in FIG. 1F. Meanwhile, a PSG layer 14 is formed on the n+ semiconductor regions 11′, and later, the PSG layer 14 is removed by an etching procedure, as shown in FIG. 1G.
Next, an aluminum conductor layer and a silver conductor layer are respectively formed on the back-lighted side S2 and the light-receiving side S1 by screen printing. Afterwards, by firing the silver conductor layer, a first electrode 15 is formed on the light-receiving side S1. Similarly, by firing the aluminum conductor layer, a back surface field (BSF) layer 16 and a second electrode 17 are formed on the back-lighted side S2, as shown in FIG. 1H, thereby completing the solar cell.
The above process forms a solar cell having selective emitters, wherein the solar cell includes emitters that are formed in different regions and have different doped concentrations and different diffusion depths. For example, the heavily doped n+ semiconductor regions 11′ are formed under the first electrodes 15 on the light-receiving side S1, and the n− semiconductor regions 11 are formed on other regions, as shown in FIG. 1H. Since the selective emitters have two different kinds of doped regions, the contact resistance between the first electrodes and the emitters can be reduced, and the electron-hole recombination rate on the surface of the solar cell can also be reduced, so as to increase the blue absorption and the photo-electric conversion efficiency of the solar cell.
The above conventional process of manufacturing the solar cell having selective emitters needs two thermal diffusion procedures to form the lightly doped regions and the heavily doped regions, respectively. However, the high temperature procedure of the thermal diffusion easily causes damage to the semiconductor structure and increases heat consumption.
In views of the above-described disadvantages resulted from the conventional process, the applicant keeps on carving unflaggingly to develop a process of manufacturing a solar cell through wholehearted experience and research.